1. Field of the Invention
This invention relates to an electrophotographic apparatus of the type in which electrostatic latent images recorded on an electrophotosensitive film are toner-developed one frame after another with a liquid developer, and more particularly to a method of quickly purging an excess of the liquid developer remaining after the process of development, without degrading the picture quality.
2. Description of The Prior Art
In an electrophotographic recording system of wet type, picture image data are recorded on an electrophotosensitive film by five sequential steps, that is, the steps of charging, exposure, developing, drying and fixing. Toner images of electrostatic latent images of picture image data are electrically held on such an electrophotosensitive film, and it is necessary to immediately fix those toner images. Therefore, various process heads have been developed in an attempt to shorten the length of time required for the image processing and to reduce the size of the individual processing sections, as disclosed in, for example, U.S. Pat. Nos. 3,972,610 and 4,461,561.
Also, a process head proposed by the inventors is disclosed in Japanese unexamined Patent Publication No. 59-100479 (corresponding to U.S. Ser. No. 578522 filed on Feb. 9, 1984), and an improved version of the proposed process head is shown in a perspective view in FIG. 1. Referring to FIG. 1, the improved process head generally designated by the reference numeral 101 includes a charging and exposure section 103 for charging and exposure of an electrophotosensitive film 102, a developing section 104 for toner-developing with a liquid developer an electrostatic latent image recorded on the electrophotosensitive film 102, a drying section 105 for drying the liquid developer on the electrophotosensitive film 102, and a fixing section 106 for fixing the toner image on the electrophotosensitive film 102, which sections are arranged in the sequential order described above.
A predetermined area of the electrophotosensitive film 102 is charged by corona discharge from a corona wire (not shown) incorporated in the charging and exposure section 103, and picture image data (not shown) are exposed to the electrophotosensitive film 102 by a recording lens 107 mounted on the process head 101 opposite to the corona wire. The electrostatic latent image formed on the electrophotosensitive film 102 as a result of exposure is toner-developed with a liquid developer in the developing section 104, and, after the liquid developer is applied to the electrophotosensitive film 102 it is exposed to a drying gas such as air supplied to the drying section 105, and the toner image is head-fixed to the electrophotosensitive film 102 by a fixing element such as a xenon lamp (not shown) incorporated in the fixing section 106.
FIG. 2 shows a developing system including such a process head 101. Referring to FIG. 2, a liquid developer 110 contained in a toner bottle 109 is pumped up by a pump 108 into a liquid developer tank 111 disposed above the process head 101, and an overflow pipe 112 extends downward from the liquid developer tank 111 so that a constant quantity of the liquid developer 110 is always held in reserve in the liquid developer tank 111. FIG. 3 is a sectional view of the developing section 104. As shown in FIG. 3 which is a sectional view of the developing section 104, a liquid supply pipe 114 extends from the liquid developer tank 111 to the process head 101 to supply the liquid developer 110 contained in the liquid developer tank 111 into a developing chamber 113 of the developing section 104 utilizing potential energy, and a flow control value 115 controlling the flow of the liquid developer 110 into the developing chamber 113 is disposed midway of the liquid supply pipe 114. Therefore, the flow rate of the liquid developer 110 supplied into the developing chamber 113 is maintained substantially constant to attain uniform development. It is apparent that a high-performance toner pump may be used to directly supply the liquid developer 110 into the developing chamber 113. As shown in FIG. 2, a blower 117 supplying compressed gas or air is connected by an air supply pipe 118 to the developing chamber 113 so that the liquid developer 110 remaining in the developing chamber 113 after termination of supply of the liquid developer 110 as well as an excess of the liquid developer 110 attaching to the electrophotosensitive film 102 can be discharged into a drain pipe 116.
During the development of the electrophotosensitive film 102, a suction slit 112 formed adjacent to the developing chamber 113 is maintained at a negative pressure through a section pipe 119 communicating with a vacuum pump (not shown) so as to suck the portion of the liquid developer 110 exuding from the developing chamber 113. Then, after pressing the electrophotosensitive film 102 against an opening 121 of the developing chamber 113 by a film pressing plate 120 to close the opening 121, the valve 115 is opened for a predetermined period of time to supply a predetermined quantity of the liquid developer 110 from the liquid developer tank 111 into the developing chamber 113 and, thence, into the drain pipe 116, thereby toner-developing the electrostatic latent image formed on the electrophotosensitive film 102. Subsequently, the gas flower 117 is driven to feed compressed air into the developing chamber 113 to purge any excess of the remaining liquid developer 110 from the surface of the electrophotosensitive film 102. As another means for supplying compressed air, the combination of a compressed air accumulator tank and a pressure regulating valve mounted midway of the air supply pipe 118 is also known.
However, as will be apparent from FIG. 3, the liquid passage in the developing chamber 113 must have a small cross-sectional area so that the liquid developer 110 flows necessarily along the surface of the electrophotosensitive film 102. As a result, immediately after the termination of supply of the liquid developer 110, a portion of the liquid developer 110 remains in the developing chamber 113 due to the capillary phenomenon. When air at high pressure is blown into the developing chamber 113 through the air supply pipe 118 under such a state, the excess of the liquid developer 110 is instantaneously jetted from the developing chamber 113 toward and into the drain pipe 116. It has been found that the toner image formed on the electrophotosensitive film 102 tends to be damaged by the jetted liquid developer 110, resulting in degradation of the picture quality attributable to, for example, non-uniform picture density.